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Description: Recent work suggests that moose (Alces alces [L., 1758]) in the boreal ecoregion may be nutritionally limited by the availability of protein as a consequence of both low nitrogen (N) concentrations and high tannin levels in their principal foods. However, the ability of moose to digest protein in the presence of tannins is unknown. We undertook a series of digestion trials with captive moose to quantify the effects of tannins and compare the digestive capabilities of moose to other browsing cervids. We fed four moose 14 single-species diets including 10 native tannin-containing forages and 4 non-tanniferous foods over four winters. True protein digestibility in moose was 92%, and metabolic fecal N excretion was estimated at 0.389 g N/100 g dry matter (DM) intake. Tannins reduced protein digestion, on average, by 38%, and the rate of reduction in protein digestion was estimated to be 8.28 g protein/100 g DM per unit protein-precipitating capacity. The digestion of protein, neutral detergent fiber, DM, and energy of tanniferous foods was not significantly different between moose and mule (Odocoileus hemionus [Rafinesque, 1817]) or white-tailed deer (Odocoileus virginianus [Zimmermann, 1780]). However, our experiments provide refined assays for evaluating the nutritional quality of browses for moose.